Add folder uploading

[Hattorius]

It should be possible to upload a folder or multiple files to cipherdrop. Currently it's possible to upload multiple files at once, but this functionality has to be changed as well. You should be able to select multiple files / folders (or singular) and they'll all be zipped together and handled like a normal file.

Expected features

• Clicking the upload button also allows folder selection
• You should be able to drop folders into the dropzone
• If multiple folders and/or files get selected they all get zipped together
• This zip should be handled just like when you upload a singular file

[Hattorius]

Currently index.js handles multi file select as follows:

const handleFiles = files => { // FileList
    for (const file of files) {
        handleFile(file);
    }
}

As you can see, it uploads every file separately. I want that zipped. Think of using the JSZip library.